Power conversion systems are used to provide AC output power to a load, such as motor drives with an inverter stage driving an AC motor. Active front end (AFE) converters employ a switching rectifier to convert input AC power to provide DC power to a bus, with the inverter switches converting the DC bus to output currents to drive the load. Such active front end converters are typically coupled with input filters, such as LCL filter circuits connected to each power phase. Since the front end rectifier is a switching circuit, the input filter operates to prevent introduction of unwanted harmonic content into the power grid or other input source. Filter components, including the filter inductors, are typically designed according to the power converter rating, where oversizing input filter components adds cost to the system and occupies valuable enclosure space. However, situations may occur in which grid voltages sag, or in which an available input source voltage is lower than the nominal AC input voltage for which the converter was designed. In certain applications, moreover, it may be desirable to operate a higher voltage motor or other load even though the source voltage is low, for instance, a 400 V input voltage to drive a 460 V motor. In these situations, the active front end rectifier can be operated in boost mode to essentially increase the gain of the front end converter, thereby boosting the DC bus voltage. At full load conditions, however, boost mode operation of the active front end rectifier leads to increased ripple and other harmonics, which can overheat the filter inductor core. One or more thermal shutoff switches may be positioned to sense the inductor temperature increase and cause a safe system shutdown. However, tripping the drive may not be desired in certain applications, and thus it is desirable to have a technique to allow the system to operate in boost mode without shutdown. In addition, such a thermal switch may be positioned some distance from the inductor core in order to sense temperature increases due to multiple causes, such as to detect whether a system blower fan is off while a full load is being driven, and thus may be unable to quickly detect overheating in the filter inductor core. Adding multiple thermal switches may address this issue, but this approach adds further cost and complexity to the system. Accordingly, there is a need for improved power converter apparatus and operating techniques to facilitate selective operation with an active front end in boost mode while mitigating or avoiding thermal stress to filter inductors.